Correlations between the home-range size of northern spotted owls (Strix occidentalis caurina) and proportion of their range in old-growth forest have been reported, but there are few data on the relationship between their home-range size and prey. The primary prey of spotted owls are wood rats and northern flying squirrels (Glaucomys sabrinus). Wood rats are larger and heavier than flying squirrels, and their population densities tend to be much greater than those of flying squirrels. We present data indicating that the home ranges of spotted owls are smaller where their diet consists predominantly of wood rats than where it consists predominantly of flying squirrels, and the proportion of the diet consisting of wood rats and flying squirrels explained significant variation in home-range size. We also found a significant correlation between home-range size and abundance of wood rats. These data indicate that prey species are a better predictor of home-range size than the proportion of older forest within spotted owl home ranges in the Klamath Province of northwestern California and southwestern Oregon, an area that is predominantly late-successional forest. Differences in habitat use were also related to prey species. Where spotted owls foraged for wood rats, the results indicated a preference for habitat edges, but where they utilized flying squirrels no such patterns were apparent.